1. Technical Field
The invention relates to the printing of color documents. More particularly, the invention relates to the printing of black text over a colored background.
2. Description of the Prior Art
Any color visible to the human naked eye can be mapped to a single point of a three dimensional space. The CIE (Centre International de l' éclairage) has normalized color representation in three-dimensional spaces. Such spaces have taken denominations such as Lab, CIE-RGB, and Luv. A good example of this color three dimensionality is the use of three phosphores in any commercial CRT (video monitor) to display a wide range of colors on a screen. In such case, the three color vectors are red, green, and blue. Similarly, and in theory, only three inks should be sufficient to render any color on a piece of white paper. Printing is an additive process. Accordingly, cyan, magenta, and yellow inks should be able to do the job of printing any color on white materials.
Unfortunately, because of chemical interactions between the colorants themselves, and because of interactions between the colorants and the printed material, it is nearly impossible to print grays consistently with only three colorants. Therefore, a fourth ink, i.e. black, is almost always added to cyan, magenta, and yellow in any printing devices. This technique is applied in the industry from traditional industrial offset printing to state of the art digital printers.
A fast growing category of color print device is that of color laser printers, which include optical and digital devices. All of these devices print color documents in four successive passes, where the four inks are first deposited in sequence onto an imaging drum which, in turn, imposes the media.
Although these digital devices can vary greatly in their specific ways of depositing colorants onto a white media, most of them are subject to color plane misregistration, where the same logical position on a page is actually mapped to different, albeit close by, physical locations on the printed materials for the four color planes. Mechanically, it is indeed very difficult for high resolution devices to have the four color planes perfectly superimposed.
One striking example where this misregistration issue is most acute is when black text is printed on top of a saturated color background. For the reasons explained above, the text is most likely to be rendered using the black colorant only, while the saturated colored background is rendered by using large amounts of cyan, magenta, or yellow colorants, or a combination of these colorants, without any or with hardly any black colorant. Because of the plane misregistrations, it is very likely that the printed sample displays a thin white border surrounding text printed on saturated colored backgrounds. As a result, black text print quality is significantly degraded.
This misregistration issue is well known in the industry and several attempts have been made to remedy it. See, for example R. Coleman, Non Uniform Modification of Process Black Colorants To Achieve Conflicting Quality Requirements, U.S. Pat. No. 5,737,088 (Apr. 7, 1998); R. Coleman, Automatic Algorithmic Determination of Process Black Over A Color Field, U.S. Pat. No. 5,784,172 (Jul. 21, 1998); N Goodman, P. Torpey, S. Harrington, B. Smith, Registration of color imagex, European Patent Application No. EP 0 833 216 (filed Sep. 22, 1997); R. Coleman, Method and system for digital color printing, European Patent Application No. EP 0 782 098 (filed Dec. 20, 1996); and R. Dermer, E. Reifenstein, Method for Determining Color Boundaries For Correcting For Plate Misregistration in Color Printing, U.S. Pat. No. 5,313,570 (May 17, 1994).
Such known techniques unfortunately spawn undesirable side effects, even if they fix the black vs. colored background misregistration problem. For example, if the technique of Coleman (EP 0 782 098) is applied, it is likely that a continuous thick black strip running on top of a white area of the page and, e.g. a yellow area of the page, show a noticeable density shift, even possibly a hue shift at the white/yellow border. See, for example, FIG. 1 which is a schematic diagram showing the printing on a page 10 of a black strip 12 on top of a yellow strip 14 according to the prior art. It can be seen that, where process black (k+y) is printed, that is where the black strip is over the yellow strip, there is an objectionable black density and hue shift.